This invention relates to the measurement of air pollutant emissions from gas turbines, boilers, process heaters, furnaces and other combustion sources. More particularly, the invention relates to a continuous emission monitoring method and system that minimizes bias in the measurement of air pollutant emissions.
Power plants and other users of combustion sources such as gas-fired turbines are required to keep emissions of certain exhaust constituents at or below specified levels. Continuous emission monitoring systems (CEMS) are required in order to assure on a continuous basis that the mandated levels are maintained. Over time, however, improved methods of minimizing emissions have evolved and emissions standards have become more stringent. CEMS systems are therefore required to consistently measure lower and lower concentrations of pollutant emissions.
As the maximum allowable concentrations of air pollutants have decreased, the effects of bias in CEMS measurements have become more and more significant. One example of particular interest is in the measurement of nitrogen oxides concentration. Nitrogen oxides emitted from combustion sources are defined as the sum of nitric oxide (NO) and nitrogen dioxide (NO2). The combination of the two is referred to as NOx. The majority of NOx from many combustion sources is NO with about 2% to 5% as NO2. For gas turbines, NO2 typically represents 5 to 15% of the total NOx. Under certain circumstances, however, in gas turbines equipped with NOx control systems, NO2 may represent as much as 50% of total NOx. In most cases, the relative amounts of NO and NO2 need not be determined. Nevertheless, the presence of a greater or lesser amount of NO2 can have an impact on bias in the measurement system used to determine the total NOx concentration.
Current CEMS systems are subject to a number of sources of bias with respect to NOx measurement. The resulting biases become more significant as the total NOx concentration level gets smaller. Recent technology improvements have resulted in combustion systems with NOx emissions in the range of 1 to 10 ppm. Environmental Protection Agency (EPA) regulations place upper limits on the sum of all measurement bias and constituent interference effects. Bias effects in the systems now in use may prevent these systems from meeting the accuracy requirements that go along with the measurement of smaller concentrations.